t.i.ssl : module documentation

SSL connections require a ContextFactory so they can create SSL
contexts. End users should only use the ContextFactory classes directly -
for SSL connections use the reactor.connectSSL/listenSSL and so on, as
documented in IReactorSSL.

All server context factories should inherit from ContextFactory, and all
client context factories should inherit from ClientContextFactory. At the
moment this is not enforced, but in the future it might be.